The present invention relates to a process for coating sintered metal carbide substrates with a diamond film.
In this context the term "sintered metal carbide" is applied to tungsten carbide sintered metal with Co bonding agent.
With respect to the need for diamond coated sintered metal carbide substrates and the development of the relevant technologies reference is made to the statements in DE-OS 19 522 372 which herewith are declared to be an integral part of the present description.
In the light of the present invention the following fundamentals of the state-of-the-art appear to be important:
From EP 0 375 923 dating from 1989 it is known that after a substrate has been machined the latter is subjected to a temperature of between 1,000.degree. C. to 1,600.degree. C. and subsequently the cobalt is removed from the substrate surface by means of a selective chemical etching step. Subsequently the surface is activated in an ultrasonic bath by means of hard particles, for example, diamond powder. The diamond film is subsequently deposited by means of a CVD process.
The results compiled in said patent application show a clear improvement in the wear behavior of the diamond coated substrate, however, indiscriminately for those cases in which nucleation with powder was performed or not. The differences in the results is based on the performance or non-performance of the thermal treatment.
EP 0 519 587 dating from 1991 describes the pre-existing approaches including those of said EP 0 374 923, that is, to "polish or score" the substrate surface prior to diamond coating, however, it does not describe how this "polishing or scoring" is performed. It does mention, however, that it had become known that without polishing or scoring no satisfactory adhesion of the diamond film to the sintered metal carbide substrate was achievable.
Said patent application now proposes to first subject the sintered metal carbide substrate surface to a selective tungsten carbide etching step and subsequently to a second etching step that removes the residues of the first etching step. The diamond film is subsequently deposited by a CVD or PECVD process. In defining the term "residue" etching step more precisely it is stated that the residues of the solution used in the first etching step as well as part of the cobalt bonding agent would be removed by the second etching step.
In any case it is emphasized that the treated substrates are not polished.
DE-OS 19 522 372 dating from 1995 finally proposes, to indiscriminately subjected polished or scored sintered metal carbide substrates first to a selective Co etching step in a first chemical system, after cleaning to a second selective tungsten carbide etching step in a second chemical system, and to finally deposit the diamond film after a second cleaning step. It is claimed that the desired removal of surface grains from the tungsten carbide could be achieved through said multi-step etching process and that the only difference in whether the substrates were previously polished or scored was that in one case the tungsten carbide grains were damaged whereas in the other they were not.
Based on the latter technique in which the sintered metal carbide substrates are subjected to a selective tungsten carbide etching step and a diamond film is subsequently deposited on the desired portion of the substrate, the task of the present invention is to create a process that further improves the adhesion of the diamond film on the sintered metal carbide substrate. This is achieved by nucleating the substrate with diamond powder after the selective tungsten carbide etching step and before the diamond film is deposited.
In contrast to EP 0 374 923 it is recognized that nucleation of the substrate surfaces with diamond powder results in significantly higher adhesive strength of the diamond film only after the selective tungsten carbide etching step. In contrast to DE-OS 19 522 372 as well as EP 0 519 587, diamond powder nucleation of the substrate surfaces result in significantly higher adhesive strength but only if this nucleation is performed after the selective tungsten carbide etching step, rather than before.